Processes and devices for friction-spinning are known in the art. According to these prior art processes, the fibers of the feeding sliver, after being opened by a fiber separation unit, are conveyed onto one or two twister elements, generally revolving or moving cylinders, provided with perforated surfaces and having a suction duct in their interior.
The suction duct defines a slot positioned along a generatrix of the cylinder. The discrete fibers are fed to the free end of the yarn being formed. They are then induced in the suction duct, by means of an air jet, exactly in correspondence with the suction slot, where they condense because of the effect of suction. While being kept adhering to the twister cylinder by the suction, they are conveyed by said cylinder. As soon as they come near the rear edge of the suction slot, and are not influenced any longer by said suction, they roll up around each other and gain twist. Thus, the fibers are condensed and twisted due to the effect of the forces caused by the suction and the motion.
The yarn which is formed thereby is then extracted along the direction of the suction slot by draw rollers and is finally wound and collected on a package.
All of such prior art spinning processes have drawbacks related to the quality of the yarn produced. In fact, the lack of proper tension on the fibers not only limits the possibility of stretching and straightening of the same fiber, but also causes the twist structure to be too spaced, so that the yarn produced is swollen and has poor strength.
Furthermore, in the prior art processes and devices the twisting of the fibers depends substantially on the dragging force applied by the motion of the twister cylinder. This dragging force is constant along a generatrix of the cylinder, and hence along the whole length of the suction slot. This results in the fibers undergoing a twisting rotation inversely proportional to the radius or thickness of fiber agglomerate. Hence, the twisting rotation is practically null at the outlet from the slot and gradually increases towards the open end of the yarn free end. Since the yarn is revolving at a high rpm value, the resulting yarn is strongly centrifuged, which causes a loss of fibers, especially of short fibers, as well as a reduction in yarn evenness. Furthermore, a high twist takes place in the yarn free end.